1. Field of the Invention
This invention relates to magnetic recording heads for reading and writing magnetic signals and, more particularly, to thin film magnetic transducers employing high permeability magnetic films for pole pieces, magnetic shields, etc. This invention also relates to any thin film magnetic transducers such as transformers, etc.
2. Prior Art
Thin film magnetic recording devices of small physical size, as reviewed, for example by Chynoweth and Kaiser, AIP Conference Proceedings 24, p. 534-540 (1974) and by Thompson, AIP Conference Proceedings 24, p. 528-533 (1974, show the effect of individual magnetic domains in response to electrical and magnetic excitation. The strength of the magnetic field required to force a magnetic film of a predetermined thickness to comprise a single magnetic domain increases approximately inversely as the linear dimensions of a structure are decreased. The latter is a demagnetizing effect which insures that an isolated magnetic film of a few hundred microns in diameter or less will contain several domains if it has a thickness of a few microns or less. When a bias field sufficiently strong to saturate the material into a single magnetic domain is applied to a film, the result is that the effective permeability is reduced to a uselessly low value. One way to overcome the problem of low effective permeability would be to make a multilayer structure of thin films if one were able to alternate the direction of the bias field from one layer to the next, since then a much smaller magnitude of bias field would be required. However, heretofore, there has been no way available or known for the purpose of producing a multiple layer thin film structure with alternating directions of bias of the thin films.
It has been known that deposition of a sandwich of Mn and NiFe layers with one layer each can sustain a magnetic bias field in the Ni-Fe film by annealing in a magnetic field at a temperature of 300.degree. C., Salanski et al, "Stabilization of Microdomain Configurations in Two-Layer Magnetic Films," Sov. Phys.-JETP, Vol. 38, No. 5, May 1974, p. 1011 et seq. A U.S. Pat. No. 3,840,898 of Bajorek et al for a "Self-Biased Magnetoresistive Sensor" teaches that a hard magnetic bias can be provided by exchange coupling between two layers if there is direct atomic contact between the layers in a magnetic recording magnetoresistive sensor. An antiferromagnetic material such as .alpha.Fe.sub.2 O.sub.3 is deposited on a glass substrate followed by deposition of Ni-Fe by evaporation in a strong magnetic field to produce a magnetically hard composite film having a permeability of 20 or less. Glazer et al in "Stabilization of the Ferromagnetic Domain Structure in Thin Films with Exchange Anisotropy, "Phys. Metals and Metallography (USSR) 26 #2, pp. 103-110 (1968) teaches stabilization of 1,000 Angstrom thick 82:18, Ni:Fe films with a 1,000 Angstrom thick manganese layer below it as the films cool through the Neel temperature following "spraying" onto a substrate. The film was demagnetized and then annealed for 11/2 hours at 350.degree. C. The exchange coupling leads to stabilization of the domain structure.
Glazer et al in "Exchange Anisotropy in Thin Magnetic Films," Soviet Physics-Solid State, Vol. 8, No. 10, pp. 2413-2420 discuss vacuum deposition of a 450 Angstrom thick manganese layer, followed by 800 Angstrom thick Ni:Fe - 82:18 layer which had uniaxial anisotropy following deposition. It was annealed in a magnetic field of 140 Oe along the easy axis at 350.degree. C. for 30 min. and cooled to room temperature in the furnace with the magnetic field still applied. The purpose of annealing was to form an antiferromagnetic layer of Mn, Fe, and Ni by mutual interdiffusion.
Massenet et al in "Magnetic Properties of Multilayer Films of FeNi-Mn-FeNiCo and of FeNi-Mn," IEEE Trans. Magnetics, MAG-1, 63-65 (1965) teach exchange coupling between an Mn film and an FeNi film in an FeNi-Mn-FeNiCo structure with Fe:Ni -- 81:19 and for an Mn layer thinner than 150 Angstroms with coupling between the magnetizations of the FeNi and FeNiCo layers. The films were prepared by evaporation in a continuous magnetic field with the easy directions of magnetization parallel in the various layers and heated to 280.degree. C., presumably to cause interdiffusion of Fe, Mn, and Ni to form an antiferromagnetic layer.
Such exchange coupling is also referred to in J. S. Kouvel, J. Phys. Chem. Solids, 24, 529 (1963).
All of the above prior art techniques require the use of annealing by heating to high temperatures to produce exchange coupling, which is undesirable for two reasons. First, it reduces permeability, i.e., increases coercivity which is highly undesirable. Second, in cases in which layering is desired and successive layers are to be biased in opposite directions, annealing of the last layer in a magnetic field would cause all layers to acquire an exchange bias field oriented in the same direction, which would exacerbate the problem of magnetic domains.
A multilayered film structure with more than two sets of exchange coupled films has not been described or taught in the prior art available. Perhaps this could be because there has not been available a technique for obtaining such a structure while maintaining high permeability. Furthermore, it would appear that the prior art has not recognized the possibility or desirability of reversing bias directions in successive sets of layers of exchange biased films.
When magnetic transducers are miniaturized to the extent that they are comparable in size to a magnetic domain, one begins to see anomalies in their electrical output which can be attributed to changes in the domain structure as the applied magnetic fields vary in strength. Troublesome discontinuous changes in sensitivity and linearity occur in the outputs of these miniaturized transducers and will be referred to as "Barkhausen noise," although the original meaning of the phrase was restricted to induced voltage spikes.
Magnetic domain effects result from changes in the position of domain walls. Hence, conceptually the effects can be eliminated by eliminating the domain walls or by making them immobile. This must be accomplished without destroying the high permeability which is required in all thin film transducer designs, and without destroying other properties, such as magnetoresistance or corrosion resistance, which are required for particular designs. Three approaches to this problem are considered below.
A first approach is to search for a material which, when used in small transducers, has no closure domains and hence no domain walls. This material must be so homogeneous and isotropic that its magnetization varies smoothly near edges to minimize the magnetostatic energy without breaking up into closure domains. Unfortunately, all known materials (including amorphous ones) have magnetic anisotropies which are large enough to cause the formation of closure domains.
A second approach is to have many walls, but to keep the magnetic excitation below the threshold for wall motion. Inductive film heads when used only for reading exemplify a situation in which excitation is quite small compared to the amount required to saturate a magnetic film. Hence, it is conceptually possible to use magnetic films with a large enough coercive force that domain wall motion never occurs during reading and high magnetic permeability results only from rotation of the magnetization within domains with fixed borders. However, all films previously made with high enough coercivity to prevent domain wall motion have had low permeability (as expected theoretically) and are thus useless to form low reluctance paths in magnetic transducers.
A third approach is to avoid domain walls by making each film a single domain. In very thin film elements such as a magnetoresistive stripe, it is possible to achieve this for certain geometries by use of an adjacent permanent magnet film (U.S. Pat. No. 3,840,898 supra) or a current-carrying conductor to produce an effective bias field. However, there are many geometries for which this is not possible. In particular, for transducers with relatively thick magnetic films (e.g., 1 to 10 .mu.m thick) such as thin film inductive recording heads, this third approach in the form of these two bias schemes is not possible. A partial solution to this problem was taught by Jean Pierre Lazzari and Igor Melnick in "Integrated Magnetic Recording Heads," IEEE Transactions on Magnetics, Vol. MAG-7, No. 1, March 1971, pp. 146-150, who showed that laminating the magnetic yoke of thin film inductive recording heads eliminates those closure domains which are magnetized perpendicularly to the easy axis. However, no provision is made by Lazzari et al to prevent multiple domains with magnetizations parallel to the easy axis and hence domain walls are still possible.
In accordance with the teachings of this application, materials, processes, and structures are provided which allow one to fabricate magnetic films, each of which is a single domain. The goal of providing single domains is achieved by use of exchange anisotropy to bias magnetic films in a unique direction which is defined during device fabrication. The use of exchange anisotropy to control magnetic domains is not, by itself, a new invention. Glazer et al supra and Salanski et al supra teach the use of exchange anisotropy to stabilize magnetic domain configurations. However, the process, materials, and structures taught by them do not pertain to thin film magnetic recording heads. Furthermore, they are not practical for fabrication and satisfactory operation of thin film magnetic transducers as discussed below.
Many attempts have been made to attain a magnetic film with unidirectional anisotropy. These attempts prior to 1971 are reviewed in A. Yelon, "Interactions in Multilayer Magnetic Films," Physics of Thin Films, Vol. 6, 1971. Basically two approaches have been taken to achieve unidirectional anisotropy.
The first approach to obtaining unidirectional anisotropy is to couple a soft ferromagnetic film weakly with a permanent magnetic film so that the direction of unidirectional bias is determined by the direction of magnetization of the permanent magnetic film. There are a number of problems limiting the practical use of this approach. One is the inability to produce high coercivity films for use as the permanent magnet in the pair which do not substantially increase the dispersion (of the direction of magentization) and lower the permeability of the soft magnetic film when the two films are placed close enough together to obtain the weak coupling required to produce unidirectional anisotropy. Another problem is to produce this weak coupling in a reproducible way. All processes known to produce this weak coupling are believed to form thin films which have pinholes between the soft and permanent magnetic film, though there is some question in the literature whether some other mechanism may be involved in a few examples. However, all of these processes have been found to give very irreproducible (highly variable) strengths of coupling and hence irreproducible amounts of unidirectional anisotropy for the soft magnetic films. For these reasons, this approach is believed to be unsatisfactory for the fabrication of practical magnetic transducers.
The second approach to obtaining unidirectional anisotropy is through the exchange interaction between an antiferromagnetic material and a ferromagnetic material as has been used here. Most of this work has been done on inhomogeneous bulk samples (very thick materials) and is not applicable to fabricating thin film devices. In thin films, there has been work on oxidizing nickel, nickel-iron, and cobalt films to produce Ni-NiO and Co-CoO interfaces. Because 80:20 NiFe is the preferred magnetic film for transducers, data pertaining to it is most relevant to the instant application. According to Bajorek [Journal of Applied Physics 46, 1376 (1975)], the oxidation of 80:20 NiFe does produce unidirectional anisotropy but the ordering temperature (defined below under Definitions) is below room temperature, and so this process is useless for practical devices since the unidirectional anisotropy would be destroyed at room temperature. Work which did produce unidirectional anisotropy in thin films with ordering temperatures above room temperature was the system NiFe-NiFeMn, where the NiFeMn is usually produced by interfacial diffusion between a NiFe film and a superimposed Mn film [O. Massenet and R. Montmory, C. R. Acad Sci. 258, 1752 (1964), O. Massenet et al supra, Glazer et al supra, and Salanski et al supra].
Attempts were made to produce unidirectional anisotropy using this technique. FIG. 1A shows the easy axis magnetization curve for a film made according to the method of Glazer et al Sov. Physics - Solid State 8 pp. 2413-2420 (1967). The films deposited were 100 A.degree. Mn and 600 Angstroms Permalloy Ni:Fe alloy on a substrate of oxidized silicon. As evaporated, the magnetic properties were similar to those of conventional Permalloy 80:20 NiFe alloy without manganese (H.sub.c = 2.4 oe, H.sub.k = 3.7 oe). FIG. 1A shows a coupling field H.sub.ex of 12 oersteds, in the absence of applied field, observed after a thermal diffusion step in the processs. FIG. 1B shows the hard axis curve for the same film. The minor loop for a reduced driving field is shown in FIG. 1C. Relative permeability is only 400. FIGS. 1A, 1B, and 1C have a horizontal scale of 20 Oe/large division and a vertical scale of 10,000 gauss for 4.pi.M.
This relative permeability is too low to be useful in a magnetic transducer. Also, the temperatures required to cause the required diffusion (1/8300.degree. C.) are in excess of what the thin film structure of many transducers can tolerate. Furthermore, there is no way to make a laminated structure with alternate NiFe films biased in opposite directions.
All of these processes for developing unidirectional anisotropy involve annealing steps to produce surface oxidation, interdiffusion, or segregation of two phases to produce an antiferromagnetic-ferromagnetic interface. This puts severe limitations on the device to withstand the temperatures involved and also on the materials to be used, since the elements in the soft ferromagnetic film must be chosen to produce the correct antiferromagnetic film or vice versa.
Bajorek et al [U.S. Pat. No. 3,84098, n Note column 2, line 13, column 4, line 16, column 6, line 53, column 7, line 29 and claims 7 and 8 where the magnetically biased material is antiferromagnetic] teach the use of exchange coupling to produce permanent magnetic bias layers for an MR stripe. They teach the use of exchange coupling between an antiferromagnetic layer such as .alpha.Fe.sub.2 O.sub.3 and a soft magnetic material such as 80:20 NiFe to produce a permanent magnetic film which is then used to bias a second soft magnetic film (the MR stripe) by magnetostatic interaction between the two magnetic films and/or by exchange coupling between the two magnetic films through an insulating layer, for example, via pinholes in the insulating layer as described below. Nowhere do they teach the deliberate exchange coupling between an antiferromagnetic film and an MR stripe itself maintaining the soft magnetic properties of the MR stripe. This is because, in that patent as discussed above, for all known processes the exchange coupling between a soft magnetic film and an antiferromagnetic film caused an increase in the coercivity of the soft magnetic film making it useless as an MR stripe (which requires low coercivity and high permeability) but making it useful as a permanent magnetic film for biasing a second soft magnetic film which would be the MR stripe as envisioned in the Bajorek et al patent. In addition, nowhere do they mention a purposeful alignment of spins in the antiferromagnetic material, which is an essential part of this invention . Without this alignment of spins in the antiferromagnet, exchange coupling with a soft ferromagnetic film produces an increase in the coercivity of the soft film, which is precisely the result Bajorek et al desired to achieve. Furthermore, no distinction was made between depositing a ferromagnetic film on top of an antiferromagnetic film and the reverse order of deposition.
On the other hand, this application teaches the importance of depositing an antiferromagnetic film onto a ferromagnetic film in the presence of a magnetic field. In the alternative, if the antiferromagnetic film is deposited first, then this application teaches that the exchange coupled films must be heated above a critical temperature defined below as the ordering temperature and allowed to cool in a magnetic field to achieve the required magnetic spin alignment in the antiferromagnetic film. This critical temperature is less than the temperature used by Salanski et al and Glazer et al supra to produce diffusion between the layers.
Magnetoresistive thin film recording heads are particularly susceptible to Barkhausen noise, not only in the magnetoresistive film itself, but also in any adjacent magnetic members used for the purpose of shielding or resolution enhancement (Thompson, supra). The methods and materials of this invention may be used for magnetic domain control in any part of these transducers. However, a recently introduced structure known as the "barber pole" magnetoresistive stripe, Kujik et al, "The Barber Pole -- a Linear Magnetoresistive Head, "IEEE Transactions on Magnetics, Vol. MAG-11, No. 5, September 1975, pp. 1215-1217, is unique in that it requires a unidirectional bias along the length of the stripe. This is because domains of opposite polarity produce electrical signals of opposite polarity; a multi-domain stripe will tend to produce no signal at all. Permanent magnet bias can be used only for extremely narrow track devices or for unshielded devices of low linear resolution. Current in the shorting bias produces useful bias only at very high current densities. Only exchange bias, as taught by this invention, can assure single domain behavior for the barber pole structure over a wide range of applications.
In accordance with this invention, a magnetic thin film structure includes a sandwich composed of layers including a first layer comprising a ferromagnetic material in direct atomic contact with a second layer of an antiferromagnetic material, the first layer having a coercivity less than about 10 Oersteds and a unidirectional magnetic bias sustained by an exchange interaction relationship with the second layer provided by alignment of spins in the antiferromagnetic material, and an exchange coupling field H.sub.e greater than the coercivity H.sub.c in the presence of said exchange interaction.
Further in accordance with this invention the antiferromagnetic material has a Neel temperature above the operating temperature of the transducer selected from the group consisting of Mn gamma phase alloys stable at room temperature in the gamma phase (face-centered cubic) and antiferromagnetic oxide including .alpha.Fe.sub.2 O.sub.3 and NiO, and more particularly the Mn gamma phase alloys are binary, ternary, and higher level alloys with elements selected from the group consisting of Fe, Co, Cu, Ge, Ni, Pt, and Rh with Mn.
Still further a third layer of a nonmagnetic material is provided in contact with the first and second layers, a fourth layer of a ferromagnetic material and a fifth layer of an antiferromagnetic material are in direct atomic contact with each other, the fourth and fifth layers being on the opposite side of the third layer, the fifth layer having a unidirectional bias direction sustained by an exchange interaction relationship with the fourth layer provided by alignment of spins in the antiferromagnetic material. Preferably the structure comprises a magnetic transducer and the nonmagnetic material includes a conductor which forms an inductive thin film magnetic sensor. It is also desirable that the second layer should be a separate, homogeneous layer having a substantial thickness as distinguished from an antiferromagnetic layer formed by diffusion.
Alternatively, the second layer is a vacuum deposited thin antiferromagnetic layer deposited while a magnetic field is applied to the first layer to provide the unidirectional anisotropy. Preferably, the first layer is an elongated narrow magnetoresistive sensor stripe, and the unidirectional magnetic bias has a substantial component along the length of the magnetoresistive sensor stripe. Furthermore, it is preferred that the second layer should have a second surface opposite from the first layer, a thin film of nonmagnetic material contacts the second surface, and a thin film of ferromagnetic biasing material is in direct contact with the nonmagnetic thin film on the opposite side thereof from the second surface. Alternatively, in the stripe embodiment a barber pole shorting metallization structure is deposited adjacent to and along the length of said stripe. In a modification, the second layer has a second surface opposite from the first layer, including a ferromagnetic thin film of low coercivity, highly permeable material in intimate exchange biasing relationship with the second surface of the second layer, with a unidirectional, magnetic anisotropy in the ferromagnetic film whereby an exchange coupled bias field is provided from the ferromagnetic thin film to the first film in a direction having a substantial component normal to the length of the elongated sensor stripe in the first layer.
In still another aspect of the basic invention, the magnetic thin film structure includes a third layer of a nonmagnetic material provided in contact with the second layer, and a fourth layer of a low coercivity, high permeability ferromagnetic material in contact with the opposite surface of the third layer having a magnetic isotropic characteristic accommodating return magnetic flux linkage from the first layer. Preferably, a fifth layer of a nonmagnetic material is in contact with the opposite surface of the fourth layer, a sixth thin layer film of a ferromagnetic material is in contact with the fifth layer and in direct atomic contact with a seventh layer of an antiferromagnetic material, the sixth layer having a coercivity less than about 10 Oersteds and a unidirectional magnetic bias sustained by an exchange interaction relationship with the seventh layer provided by alignment of spins in the antiferromagnetic material and an exchange coupling field H.sub.e greater than the coercivity H.sub.c in the presence of the exchange interaction.
In an entirely different aspect of the invention a method is provided for forming a magnetic thin film structure comprising depositing a first film of a low coercivity, highly permeable ferromagnetic material upon a substrate, and depositing a second layer of an antiferromagnetic material in direct atomic contact with the first film while maintaining a magnetic field upon the first film.
Alternatively a method of forming a magnetic thin film structure comprises depositing a pair of films upon a substrate, with a first one of the films comprising a ferromagnetic low coercivity (below 10 Oersteds) highly permeable material upon a substrate, a second one of the films comprising an antiferromagnetic material, the films being in direct atomic contact with each other, and subsequently heating the films above the ordering temperature while maintaining a unidirectional magnetic field applied to the films during cooling thereof producing an exchange interaction relationship between the films with alignment of spins in the antiferromagnetic material and an exchange coupling field H.sub.e greater than the coercivity in the presence of said exchange interaction while retaining low coercivity below 10 Oersteds in the first film.
In still another aspect of the invention a composition of matter is provided comprising a first material having a low coercivity of less than 10 Oersteds in direct atomic contact with a second material which is antiferromagnetic, with an exchange interaction relationship between the materials provided by alignment of spins in the antiferromagnetic material, and an exchange coupling field H.sub.e greater than the coercivity H.sub.c in the presence of the exchange interaction.
In general all of the features are adapted for use in thin film magnetic recording heads.
In accordance with this invention, a magnetic thin film transducer includes a sandwich composed of a plurality of layers including a first pair of layers including a layer of soft magnetic material in direct contact with a layer of antiferromagnetic material wherein the soft material has a unidirectional bias direction sustained by an exchange interaction relationship with the antiferromagnetic material, a nonmagnetic layer deposited upon the first pair of layers, a second pair of layers including a second layer of a soft magnetic material in direct contact with a second layer of antiferromagnetic material wherein the second layer of soft material has a unidirectional bias direction sustained by an exchange interaction relationship with the second layer of antiferromagnetic material.
Further in accordance with this invention, the unidirectional bias directions of the first and second pairs of layers are in opposite directions.
In another aspect of this invention, the second pair of layers is replaced by a single ferromagnetic layer.
In still another aspect of this invention, the nonmagnetic layer comprises a thin film conductor for an inductive magnetic recording head, wherein the second pair of layers is as originally stated.
In still another aspect of the invention, a barber pole structure is biased by a ferromagnetic layer exchange biased by an antiferromagnetic material.